ABSTRACT

Aerated concrete is relatively homogeneous when compared to normal concrete, as it

does not contain coarse aggregate phase, yet shows vast variation in its properties. The

properties of aerated concrete depend on its microstructure (void-paste system) and

composition, which are influenced by the type of binder used, methods of pore-

formation and curing. Although aerated concrete was initially envisaged as a good

insulation material, there has been renewed interest in its structural characteristics in

view of its lighter weight, savings in material and potential for large scale utilisation

of wastes like pulverised fuel ash. The focus of this article is to classify the

investigations and their effect on the thermal properties of aerated concrete.

1. INTRODUCTION:-

Aerated concrete is either a cement or lime mortar, classified as lightweight concrete,

in which air-voids are entrapped in the mortar matrix by means of a suitable aerating

agent. Broadly speaking aerated concrete falls into the group of cellular concrete

(microporite being the other). The prominent advantage of aerated concrete is its

lightweight, which economises the design of supporting structures including the

foundation and walls of lower floors. It provides a high degree of thermal insulation

and considerable savings in material due to the porous structure. By appropriate

method of production, aerated concrete with a wide range of densities (300-1800

kg/m3) can be obtained thereby offering flexibility in manufacturing products for

specific applications (structural, partition and insulation grades). There have been

several investigations on the properties of aerated concrete in the past.

Although aerated concrete was initially envisaged as an

insulation material, there has been renewed interest on its structural characteristics in

view of its lighter weight, savings in material and potential for large scale utilisation

of wastes like pulverised fuel ash.

1

In the mold process, the mix slurry generates hydrogen gas by the chemical reaction

between fine aluminium powder and lime.

2. Classification of aerated concrete

2.1. Based on the method of pore-formation

AIR-ENTRAINING METHOD (GAS CONCRETE): Gas-forming chemicals are

mixed into lime or cement mortar during the liquid or plastic stage, resulting in a mass

of increased volume and when the gas escapes, leaves a porous structure. Aluminium

powder, hydrogen peroxide/bleaching powder and calcium carbide liberate hydrogen,

oxygen and acetylene, respectively. Among these, aluminium powder is the most

commonly used aerating agent. Efficiency of aluminium powder process is influenced

by its fineness, purity and alkalinity of cement, along with the means taken to prevent

the escape of gas before hardening of mortar. In the case of Portland cements with low

alkalinity, addition of sodium hydroxide or lime supplement the alkali required.

FOAMING METHOD (FOAMED CONCRETE): This is reported as the most

economical and controllable pore-forming process as there are no chemical reactions

involved. Introduction of pores is achieved through mechanical means either by pre-

formed foaming (foaming agent mixed with a part of mixing water) or mix foaming

(foaming agent mixed with the mortar). The various foaming agents used are

detergents, resin soap, glue resins, saponin, hydrolysed proteins such as keratin etc.

Combined pore-forming method: Production of cellular concrete by combining

foaming and air-entraining methods has also been adopted using aluminium powder

and glue resin.

2.2. Based on the type of binder

2

Aerated concrete is classified into cement or lime based depending on the binder used.

Attempts have also been made to use pozzolanic materials such as pulverised fuel ash

or slate waste as partial replacement to the binder or sand.

2.3. Based on the method of curing

Aerated concrete can be non-autoclaved (NAAC) or autoclaved (AAC) based on the

method of curing. The compressive strength, drying shrinkage, absorption properties

etc. directly depend on the method and duration of curing. The strength development

is rather slow for moist-cured products. Autoclaving initiates reaction between lime

and silica/alumina bearing ingredients. The hydrothermal reactions involved in

autoclaving have been explained.

Carbonation is one of the main degradation mechanisms that possibly affect the

durability and service life of concrete materials in general. Carbonation occurs when

portlandite [Ca(OH)2] and other calcium-bearing C-S-H phases react with CO2 to

form calcite (CaCO3). Major mineralogical changes associated with carbonation

include conversion of calcium silicate hydrate gel, in this case, tobermorite:

Degradation process is further accelerated by high concentrations of sulphur dioxide

in the air. SO2 in the presence of moisture forms sulphurous acid. By atmospheric

oxygen, sulphurous acid is oxidised to sulphuric acid, which attacks calcium

carbonate. Gypsum crystals occur as a reaction product.

3

Thermal Property

Thermal property/conductivity depends on density, moisture content and ingredients

of the material. As Thermal conductivity is largely a function of density, it does not

really matter whether the product is moist cured or autoclaved as far as thermal

conductivity is concerned. The amount of pores and their distribution are also critical

for thermal insulation. Finer the pores better the insulation. As the thermal

conductivity is influenced by the moisture content (a 1% increase in moisture by mass

increases thermal conductivity by 42%), it should not be reported in oven dry

condition. In order to obtain high thermal insulation and compressive strength, non-

crystalline siliceous material has been adopted by some researchers as a partial or full

replacement for quartz sand.

Methods used in analysis of AAC:

XRD ANALYSIS

X-ray diffraction (XRD) is a versatile, non-destructive technique that reveals detailed

information about the chemical composition and crystallographic structure of natural

and manufactured materials.

Crystal lattice

A crystal lattice is a regular three-dimensional distribution (cubic, rhombic, etc.) of

atoms in space. These are arranged so that they form a series of parallel planes

separated from one another by a distance d, which varies according to the nature of the

material. For any crystal, planes exist in a number of different orientations - each with

its own specific d-spacing. 

Constructive interference

When a monochromatic X-ray beam with wavelength lambda is projected onto a

crystalline material at an angle theta, diffraction occurs only when the distance

travelled by the rays reflected from successive planes differs by a complete

number n of wavelengths.

4

XRD MACHINE

SEM ANALYSIS

The scanning electron microscope (SEM) uses a focused beam of high-energy

electrons to generate a variety of signals at the surface of solid specimens. The signals

that derive from electron-sample interactions reveal information about the sample

including external morphology (texture), chemical composition, and crystalline

structure and orientation of materials making up the sample. In most applications, data

are collected over a selected area of the surface of the sample, and a 2-dimensional

image is generated that displays spatial variations in these properties. Areas ranging

from approximately 1 cm to 5 microns in width can be imaged in a scanning mode

using conventional SEM techniques (magnification ranging from 20X to

approximately 30,000X, spatial resolution of 50 to 100 nm). The SEM is also capable

of performing analyses of selected point locations on the sample; this approach is

especially useful in qualitatively or semi-quantitatively determining chemical

compositions (using EDS), crystalline structure, and crystal orientations

(using EBSD).

Fundamental Principles of Scanning Electron Microscopy (SEM)

5

Accelerated electrons in an SEM carry significant amounts of kinetic energy, and this

energy is dissipated as a variety of signals produced byelectron-sample

interactions when the incident electrons are decelerated in the solid sample. These

signals include secondary electrons (that produce SEM images), backscattered

electrons (BSE), diffracted backscattered electrons (EBSD that are used to determine

crystal structures and orientations of minerals), photons (characteristic X-rays that are

used for elemental analysis and continuum X-rays), visible light

(cathodoluminescence--CL), and heat. Secondary electrons and backscattered

electrons are commonly used for imaging samples: secondary electrons are most

valuable for showing morphology and topography on samples and backscattered

electrons are most valuable for illustrating contrasts in composition in multiphase

samples (i.e. for rapid phase discrimination). X-ray generation is produced by inelastic

collisions of the incident electrons with electrons in discrete ortitals (shells) of atoms

in the sample. As the excited electrons return to lower energy states, they yield X-rays

that are of a fixed wavelength (that is related to the difference in energy levels of

electrons in different shells for a given element). Thus, characteristic X-rays are

produced for each element in a mineral that is "excited" by the electron beam. SEM

analysis is considered to be "non-destructive"; that is, x-rays generated by electron

interactions do not lead to volume loss of the sample, so it is possible to analyze the

same materials repeatedly.

6

SEM MACHINE

TGA ANALYSIS

Thermogravimetric analysis or thermal gravimetric analysis is a type of testing

performed on samples that determines changes in weight in relation to

a temperature program in a controlled atmosphere. Such analysis relies on a high

degree of precision in three measurements: weight, temperature, and temperature

change. As many weight loss curves look similar, the weight loss curve may require

transformation before results may be interpreted. A derivative weight loss curve can

identify the point where weight loss is most apparent. Again, interpretation is limited

without further modifications and deconvolution of the overlapping peaks may be

required. To determine composition and purity one must take the mass of the

substance in the mixture by using thermal gravimetric analysis. Thermal gravimetric

analysis is the act of heating a mixture to a high enough temperature so that one of the

components decomposes into a gas, which dissociates into the air. It is a process that

utilizes heat and stoichiometry ratios to determine the percent by mass ratio of a

solute.

Methodology

TGA is a process that utilizes heat and stoichiometry ratios to determine the percent

by mass of a solute. Analysis is carried out by raising the temperature of the sample

gradually and plotting weight (percentage) against temperature. The temperature in

many testing methods routinely reaches 1000°C or greater. After the data are

obtained, curve smoothing and other operations may be done to find the exact points

of inflection. A method known as hi-resolution TGA is often employed to obtain

greater accuracy in areas where the derivative curve peaks. In this method,

temperature increase slows as weight loss increases. This is to more accurately

identify the exact temperature where a peak occurs. Several modern TGA devices can

vent burn off to an infrared spectrophotometer to analyze composition.

7

8

LITERATURE REVIEW

In this section some major research works done in the field of thermal property of

AAC will be summarised and discussed.

Case 1:

Analysis of Microstructure and Properties of Autoclaved Aerated Concrete Wall

Construction Materials [15]

This work sets its aim at examining the porous structure of both AAC and Non-AAC

in order to elucidate the formation process of their microstructures using optical

microscopy and scanning electron microscopy (SEM). Also chemical and structural

analyses are conducted using X-ray diffraction (XRD) to Figure out the effect of their

composition on their thermal properties. Comparison of thermal and mechanical

properties of AAC and Non-AAC has also been reported.

Studies on thermal inertia of concrete blocks are involving with heat transfer by

radiation to buildings made of AAC and Non-AAC. It was found that Non-AAC

blocks with higher density are likely to be able to transfer heat faster than the lighter

AAC and in turn lead to shorter delay time in heat transmission.

The experimental results showed that the wall made of AAC could resist moisture and

reduce heat transfer due to its inner porous structure.

Materials and Method

Specimens of AAC were collected which has been certified by Thailand Industrial

Standard (TIS) 1505-2541. 120 pieces of AAC blocks with a uniform size of 200 ×

600 × 75 mm were randomly sampled for investigation. Meanwhile, the Non-AAC

9

blocked of the same size and amounts were sampled for comparison. Both AAC and

Non- AAC specimens were subjected to tests as follows;

1) Thailand Industrial Standard (TIS) 1505-2541 and Din 4165-1986 for determining

dry density, compressive strength, and flexural strength,

2) JIS A 2618 standard for examining thermal conductivity using Thermal

Conductivity Tester (model 88 K-FOTOR, ANACON),

3) DSC analysis for determining specific heat capacity using a differential scanning

calorimeter (Model DSC 7),

4) Microscopic analysis using scanning electron microscopy (SEM, Model JEOL JSM

5800 equipped with

Energy Dispersive Spectroscope),

5) Chemical analysis using XRD (X-ray Diffractometer, model Bruker D8).

Discussion

From the SEM investigation on the surface morphology of the two typical samples of

light weight AAC and Non- AAC blocks, it could clearly be observed that the surface

Of Non-ACC sample consisted of large pores while the ACC sample exhibited

smoother surface with much smaller porosity as shown in Figure 2. For further

investigation on the porous structure of typical samples of Non-AAC and AAC, SEM

micrographs with higher magnification shown in Figure 3 are taken into account. In

Figure 3(a) Non-AAC samples consist of particulate clusters of evenly distributed

sizes, which situate around craters generated by the existence of uncontrollable air

bubbles in curing process. It could further be observed that the crystallinity of the

Non-AAC was resulted from random formation of gas bubbles, leading to formation

of cavities with various depths.

On the other hand, Figure 3(b) revealed the samples of AAC

exhibit smoother surface with some porosity and particulate of narrower size. This

could be implied that under high pressure and temperature in autoclaving process,

denser phase of C-S-H could be formed. For further investigation of their crystallinity,

SEM micrographs with further higher magnification were employed.

10

Figure 4(a) clearly reveals that Non-AAC samples are

comprised of particulate calcium silicate of unevenly distributed morphologies. Sub-

micron particulates gathering as agglomerates with average size of 5 μm could form

loose matrix surrounding by voidage, resulting in lower density and thermal

conductivity of the Non-AAC samples.

However, considering Figure 4(b), one could clearly observe

considering Figure 4(b), one could clearly observe that finer needle-like crystalline

structures exist thoroughly the surface of AAC samples. Tremendous amount of finer

porosity within the matrix of fine particulate would also result in much lower specific

density of AAC samples. Difference in such crystal morphologies and phases in

Non-AAC and AAC samples are supposed to exert significant effects on not only

mechanical but also thermal characteristics of the samples.

Table 1 summarizes the mechanical and thermal

investigating results. It could be clearly observed that AAC samples exhibit specific

density almost 20 percent lower than that of Non-AAC. On the other hand, it could

also be seen that average thermal conductivity and heat capacity of AAC samples are

significantly lower than those of Non-AAC. As could be confirmed by microscopic

analyses, the existence of micro- sized porosity in AAC samples would lead to higher

insulating performance.

11

(a)

(b)

Figure 2. SEM micrographs of non-AAC and AAC specimens.

(a)

12

(b)

Figure 3. Comparison of surface morphology of Non-AAC and AAC specimens.

(a)

13

(b)

Figure 4. Comparison of crystalline structure of Non-AAC and AAC specimens.

In order to confirm the crystal structure of both Non-AAC and AAC samples, XRD

analysis was also carried out. XRD patterns in Figure 5 reveals that Non-AAC

samples mainly consist of calcite and quartz. Meanwhile, in AAC sample main

product is crystalline in as much as tobermorite and quartz are present.

Figure 5. XRD patterns of (a) Non-AAC and (b) AAC specimens

(Q: quartz , T: tobermorite and C: calcite).

14

Conclusion

Under a condition of high pressure and temperature treatment, calcite in aerated

cement specimens could undergo phase transformation process to become

tobermorite, which is more stable than former phase. High pressure autoclaving could

also control formation of evenly distributed micropores within the cement matrix. The

existence of tobermorite phase in AAC plays an important role in providing various

outstanding properties. Additionally, AAC could exhibit superior compressive

strength and thermal insulating properties.

15

CASE 2:

Thermal Insulation and Strength of Autoclaved Light Concrete [14][15]

The objective of this research was to study the effect of the major components of ALC

on aerating process, improving the compressive strength while retaining excellent

thermal insulation simultaneously.

At the same time, relationship between thermal

insulation and compressive strength was studied when silica fume was partly replaced

by quartz sand.

Materials

Silica fume: Silica fume, also known as micro silica, is an amorphous (non-

crystalline) polymorph of silicon dioxide, silica. It is an ultrafine powder collected as

a by-product of the silicon and ferrosilicon alloy production and consists of spherical

particles with an average particle diameter of 150 nm.

Sample

In experiments, silica fumes with dominant size of 0.15-0.2 μm were used to replace

quartz sand at a dosage of 50.5 wt%. Samples of different bulk density were produced

via the change of the weight fraction of aluminium powder. Silica fumes were mixed

with water before stirred with other materials.

Fraction of aluminium powder was varied to investigate

the relationship of compressive strength and thermal coefficient. By analyzing the

evolution of compressive strength and thermal coefficient, the relationship could be

obtained.

The samples for thermal insulation and compressive strength test

were manufactured at the dimension of 300×300×30 mm and 100×100×100 mm,

respectively. They were first cured at 34oC under normal pressure for 12 hours.

Afterwards they were autoclaved under saturated steam pressure of 1.25 MPa for 8 h

16

Discussion

λ＝0.0002d＋0.0233 (1)

Where, λ is thermal coefficient, W/ (m·k); d is Dry density, kg·m－3.

Figure 1: Thermal coefficient vs dry bulk density

Eq.1 and Fig.1 showed the linear relationship between dry bulk density and thermal

coefficient. They indicated that the samples had obtained a very low thermal

coefficient as their dry bulk density had been controlled into a low extent.

Different locations of samples including wall

surface of aerated bubble, fracture section of aerated bubble were investigated by

SEM.

Fig.2 (a) and (b) showed that laminar crystal about 3 μm to 5 μm

width was found at the wall surface of aerated bubble. The surface materials of the

aerated bubble were tobermorite crystals. Nevertheless, the laminar crystals in this

17

study were denser than that Non silica fume AAC. Therefore, the mechanical

properties were improved. Fig.2 (c) and (d) revealed that the morphology of fracture

section was grainy CSH(I).

Figure 2: Microstructure of samples at different locations

The average elemental compositions of the sample

analyzed by SEM & EDX were given in a spectral diagram. Fig.3 illustrated intensity

peaks for calcium, silicon and oxygen atom, which were the major elements

18

constituting the CSH(I) gel and tobermorite crystal. Aluminium element was also

presented. Table 1 revealed the EDX data corresponding to the spectrum given in

Fig.3.

Fig.4 EDX spectra of selected targets and the location of the SEM image

Table 1: EDX data for spectrum in figure 3

The X-ray powder patterns of ALC samples showed distinct

peaks of SiO2, CSH(I), tobermorite and calcite. These results suggested that the phase

compositions of samples with XRD analysis were identical with SEM & EDX.

19

Figure 4: X-ray powder pattern of ALC samples

By SEM, EDX and XRD analyses, the surface

morphology at the wall surface of aerated bubble and fracture section of aerated

bubble could be affirmed as tobermorite crystal and CSH(I) gel, respectively.

Conclusions:

The properties of samples had been improved by optimizing the recipe. Dry bulk

density of the samples had been lowered to 284.5 kg/m3 with the compressive

strength still kept above 2.00 MPa. If the lower compressive strength was allowed, i e,

1.5 MPa or 1.0 MPa, the bulk density could even be decreased further (i e, the thermal

coefficient could be even improved further).

20

CASE 3:

Influence of air-cooled slag on Thermal properties of autoclaved aerated

concrete [1] [15] [20]

This investigation includes the study of the possibility of using the air-cooled slag by-

products as substituent for sand and lime in autoclaved aerated concrete. The

influences of slag substitution on the physico-mechanical and chemical properties of

the final products under different autoclaving time were studied.

Air Cooled Slag:- Air-cooled blast-furnace slag is a lump ore manufactured from

molten from blast-furnace slag by air-cooling and appropriate water spraying. The

product is then crushed and screened in accordance with the specified application and

sold as materials for road base course material, rock fibres, coarse concrete aggregates

and cement clinker raw materials.

Experimental Mixes

Mix Sand Cao PC As

AS0 65 25 10 --

AS1 55 25 10 10

AS2 35 25 10 30

AS3 15 25 10 50

AS4

AS5

AS6

65

55

35

10

10

10

5

5

5

20

30

50

Discussion

X-ray diffraction analyses were carried out to identify the crystalline solid phases. The

diffraction patterns are given in Figs. 1 and 2 for samples cured for 2 and 24 h,

21

respectively. Fig. 1 shows that the peaks characteristic of tobermorite were started to

appear in the reference sample (AS0) as with the low-lime samples (AS4–AS6), and

the intensity increase with increasing AS contents. This indicates that tobermorite can

be formed at 2 h of curing time in the presence of AS.

However, increasing the autoclaving time to

24 h in slag containing samples made with 10% CaO has no effect on the

tobermorite’s peaks intensity. This indicates that tobermorite formation was inhabited

after 2 h of autoclaving in these mixes.

Fig. 1. XRD patterns of autoclaved aerated concrete at 2 h of curing times.

22

Fig. 2. XRD patterns of autoclaved aerated concrete at 24 h of curing times.

The fracture surface of all samples autoclaved for 2 and 24 h are examined with

scanning electron microscopy. Fig. 3a and b shows the change in microstructure in the

reference mix (AS0). The initially formed amorphous CSH and platy like tobermorite

(Fig. 3a) change to crumbled foiled tobermorite (Fig. 3b) with further processing.

In high-lime mixes made with 10 and 30%AS substitution (AS1 and AS2), at 2-h

processing time, only fibers CSH are formed. At 24-h processing time, these CSH

fibers change to a very crystalline needle-like tobermorite. Fig. 4 is given as example

of this change for sample AS2.

Fig. 5 shows no tobermorite formed in mix AS3 (50%

AS substitution) at all autoclaving times. Rather, amorphous Al substituted CSH

formed (Fig. 5a), and it changed to reticulated amorphous CSH (Fig. 5b) with

increasing processing time. This seems to be due to the increase in the bulk C/S ratio

23

beyond those suitable for tobermorite formation. In AS-substituted mixes made with

low-CaO contents (10%), tobermorite appears at 2-h processing time. However, with

further processing, tobermorite formation was retarded and CSH with a grass-like

structure, and a very low C/S ratio was formed. Figs. 6 and 7 is the SEM picture for

mix AS5 and AS6, respectively

Fig. 3&4. SEM micrographs of the reference mix (AS0) and (AS2 )at (a) 2 and (b) 24 h.

24

Fig. 5&6. SEM micrographs of mix (AS5) and (AS5) at (a) 2 and (b) 24 h.

25

Fig. 7. SEM micrographs of mix (AS6) at (a) 2 and (b) 24 h.

Conclusion

The existence of tobermorite phase in AAC plays an important role in providing

various outstanding properties. The tobermorite phase present enhances the thermal

property of the AAC and by adding some amount of Air slag along with properly

monitored levels of CaO can help formation of toblomorite phase within two hours of

curing.

CASE 4:

Effect of high temperature and cooling conditions on aerated concrete

properties [15] [7] [8]

Concrete is an inorganic material and high temperature and its duration decrease the

concrete strength and its durability. Fire resistance of concrete is primarily affected by

factors like the temperature duration and condition (multiple or one-way effect-direct

26

flame contact-hot gas or radiation) of the fire. The type of aggregate and cement used

in its composition, the porosity and moisture content of concrete, its thermal

properties, and sizes of structure members and their construction type are the other

factors that determine the level of fire resistivity of the material.

The effect of high temperatures on the mechanical properties and

durability of lightweight, normal, or high strength concrete have been investigated by

many researchers in order achieve fire resistant material since the 1940s.

Material used

Aerated concrete tested in the experimental study was provided directly by the

manufacturer. The commercial name of the product is called G4. The material

properties are given in Table 1

Discussion

Effect of high temperatures on volume and unit weight

The linear thermal expansion coefficient of aerated concrete is a = 8 x 10-6 m/m oC.

Accordingly, material expands initially, but shrinks relatively more than its original

size depending on the applied temperature. The trend of the relational line is adjacent

to the calculated theoretical values of linear and volumetric thermal expansion of the

27

material until 200 oC which are also shown in the same figure. Dimensional changes

directly affect the volume of the material. It gradually decreases between 400 and 800 oC. Between 200 and 400 oC and above 800 oC the decrease is relatively sharp.

Thus, aerated concrete can be well accepted

as a dimensionally stable material especially up to 800 oC. Parallel to the changes in

volume, unit weight of aerated concrete decreases approximately 10% depending on

the elevated temperatures and is relatively smaller than gypsum based paste and

mortars. This value is important for the evaluation of MoE of the material.

Effect of high temperatures on the young’s modulus of elasticity

Equilibrium moisture content of aerated concrete depending on the relative humidity

of the test environment is generally below 3.5% by volume of the dry material and is

released at approximately 100 oC. From 200 oC up to 800 oC the values gradually

decrease, and remain almost same upwards. Between 300 oC and 500 oC water of

crystallization removes from the material and free calcium hydroxide decomposes into

calcium oxide CaCO3 exists in the structure of aerated concrete is more resistant to

high temperatures than Ca(OH)2, Additionally, aerated concrete does not contain

coarse aggregate and has lower thermal conductivity than NWC which may relatively

decrease the effect of quenching on the structure of the material.

MoE values also decrease gradually between 100

and 800 0C which could be the result of gradual increase of micro cracks in the

structure of the material.

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Effect of high temperatures on the compressive strength

Chemical structure of the aerated concrete starts to break down between 225 and 450 0C where first capillary cracks were observed. These cracks do not have significant

affect on the compressive strength of the material, but above 500 0C the amount of the

cracks develop parallel to the increase of temperature and the material becomes

weaker. Maximum compressive strength is found at approximately 200 0C series

which were tested hot. Starting from approximately 400 0C up to 800 0C compressive

strength of the material is gradually lost, but above 800 0C severe lost is seen due to

the sintering reaction starts to occur in the structure.

Like the evaluations on MoE, gypsum mortars lose

approximately 80% of their compressive strength at 200 0C due to the dehydration of

the matrix phase.

Conclusion

After fire exposure, the volume of the aerated concrete increases at first due to the

thermal expansion of the material, it increasingly shrinks depending on the rising

temperature level. Both the unit weight and the volume of the material decrease

approximately 10% at the examined temperature range.

High temperature does not have a

considerable effect on the strength of the material at approximately 700–800 0C.

Above this level, the strength of the material is distinctively influenced.

As studied we can see that AAC displays

much better resistance to high temperature than normal concrete due to its low thermal

conductivity.

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Case 5:

Compressive strength, flexural strength and thermal conductivity of autoclaved

concrete block made using bottom ash as cement replacement materials [4][6][15]

Bottom ash (BA) is mostly used as a partial sand replacement of concrete production

and as a substitution of fine aggregates in lightweight concrete but it does not

improved compressive strength due to it is porous particle structure and high water

absorption. Thus, the purpose of this research was to investigate the compressive

strength and the thermal conductivity of the autoclaved concrete using bottom ash. BA

was used to replace part of Portland cement at 10%, 20% and 30% by weight,

respectively. All specimens were cured by autoclave under high-pressure stream

curing. Compressive strength and thermal conductivity tests were then carried out.

30

Discussion

The thermal conductivity results of BLWC can be seen as shown in Fig. 1.

31

The addition of Al can be seen to significantly affect the thermal conductivity of the

concrete. A reduction from 0.89 to 0.57 W/m K can be observed. The thermal

conductivity was found to increase with increase BA replacement of Portland cement

due to the BA content increased the overall unit weight (bulk density) of the concrete.

The thermal conductivity depends upon the pore structure of the lightweight

aggregates, density of concrete and the cement matrix.

The decrease in thermal conductivity is due to the

decreasing of concrete density. The thermal conductivity is therefore known to

increase with the increase in the density of LWC. The thermal conductivity of LWC at

BA replacement at 10%, 20% and 30% by weight (BA1,BA2,BA3) are 0.58, 0.59 and

0.61 W/m K, respectively.

The thermal conductivity of BLWC tends to

increase with increasing BA replacement. This is due to tobermorite phase was

formed when using BA replacement, compared to a-C2SH phase in LWC without BA

replacement (PC). Tobermorite phase is responsible for the increased thermal

conductivity of BLWC due to it is a stronger product than a-C2SH phase which result

to decrease in porosity and denser structure. Moreover, its formation in preference to

anomalous tobermorite is favored by short time, low temperature, high Ca/Si ratio and

presence of Al3+ in absence of alkali. Thus, when using BA as cement replacement at

this curing temperature would increase the opportunity for tobermorite formation.

Case 6:

Influence of zeolite additive on properties of autoclaved aerated concrete [5][15]

The aim of this work was to investigate the effect of zeolite as siliceous material

replacing quartz sand in AAC.

Zeolite: The classical definition of a zeolite is a crystalline, porous aluminosilicate.

However, some relatively recent discoveries of materials virtually identical to the

classical zeolite, but consisting of oxide structures with elements other than silicon

and aluminum have stretched the definition. Most researchers now include virtually all

32

types of porous oxide structures that have well-defined pore structures due to a high

degree of crystallinity in their definition of a zeolite.

Discussion

Zeolite-based AAC (ZAAC) samples were prepared at the oven-dry unit weight of

270–500 kg/m3 by controlling the quantity of Al used as foaming agent.

Changes in thermal conductivity values vs. dry bulk

density of ZAAC and commercial AAC are given in Fig 1. The thermal conductivity

values are reduced with decrease of bulk density of these materials. Similar thermal

conductivity values were observed for ZAAC and AAC products of the same density.

XRD analyses (Fig. 2) show the presence of tobermorite, quartz and anhydrite in

commercial AAC and formation of wollastonite as a new phase at 8500C. The ZAAC

contain unreacted anhydrite and clinoptilolite phases besides tobermorite and quartz

(Fig. 3). XRD patterns obtained after treatment of the product between 700 and

10000C show that wollastonite, gehlenite and orthoclase are newly formed phases.

Fig 1: Thermal conductivity of investigated concrete vs. dry bulk density.

33

Fig 2: XRD pattern of G-2 grade AAC at 25 1C and 850 1C.

34

Fig 3: XRD patterns of ZAAC at 25 1C and 700–1000 1C.

Conclusion

Use of zeolite to replace quartzite in production of AAC provides the thermal

conductivity values similar to conventional AAC at the same bulk density.

Commercial AAC is mainly composed of tobermorite,

quartz and anhydrite, but ZAAC contains unreacted anhydrite and clinoptilolite phases

besides tobermorite and quartz.

35

Case 7:

Properties of the autoclaved aerated concrete produced from coal bottom ash [4] [6] [11]

Coal bottom ash and fly ash are quite different physically, mineralogically, and

chemically. Bottom ash is a coarse, granular, incombustible by-product that is

collected from the bottom of furnaces that burn coal for the generation of steam, the

production of electric power, or both. Bottom ash is coarser than fly ash, with grain

sizes spanning from fine sand to fine gravel. The type of by-product produced depends

on the type of furnace used to burn the coal.

BA is often used as a low-cost replacement for more expensive sand for concrete

production and as a fine aggregate in high-performance lightweight concrete. A

previous study confirmed that the pozzolanic activity of the BA could be improved

with adequate grinding. It should also be noted, however, that grinding BA resulted in

a prolonged settling time and caused a reduction on the workability of the cement

pastes.

In this study, the received BA was first pre-treated by crushing and

particle size classification method to decrease its higher unburned carbon to desirable

level or that generally accepted by concrete industry (<6%). The pre-treated ash was

then used as an aggregate with different replacement amounts.

Thermal insulation properties of BAC specimens were determined by thermal

conductivity test. The measured thermal conductivities for BAC specimens cured for

18h are given in Table 1. Thermal conductivity values reduced with increasing BA

replacement ratio. Although the lowest conductivity value was obtained for the 100%

BA replacement amount, the decreased strength values of this concrete limited the

usability.

The obtained conductivity for the specimen having 50% BA and 0.2826W/mK was

relatively higher than the commercial AAC produced, 0.2443W/mK; however, it can

be acceptable for the general classification reported for the AAC

36

Table 1: Thermal conductivity values of specimens

Micro structural analyses:

As the percentage of BA in the replacement increased, the AAC structure became

more and more porous, with more pores distributed around the BA aggregate surfaces.

In addition, the detachments of the grains were observed in the network structure with

the increase of BA replacement. Therefore, the formation of discrete grains and porous

area close to the aggregate surfaces might be the main cause in reducing the thermal

conductivity values with an increase in the BA replacement ratio against quartz sand.

Conclusion:

Utilization of porous BA in AAC mixture decreases the thermal conductivity values up to 39% when compared with the reference AAC specimen.

37

CASE 8:

Workability, and mechanical, acoustic and thermal properties of lightweight aggregate

concrete with a high volume of entrained air [10][14]

Air voids were introduced in cement matrix either by adding preformed foam,

separately produced in a foam generating machine or by an air entraining (AE) agent

during the mixing process. In general, the former method was preferred because it was

easier to reach required air content in concrete. On the contrary, our previous studies

have been focused on the lightweight aggregate concrete foamed by AE agent because

a foam generating machine was unnecessary. These studies were about the effect of

types and sizes of lightweight aggregates, and the addition of various fibers and

mineral admixtures on the mechanical properties of lightweight aggregate concrete

with adequate amount of AE agent.

Discussion:

Fig. 1 shows that the thermal conductivity of lightweight concrete is lower than that of

normal concrete. While the thermal conductivity of normal concrete was 0.7–0.8 W/m

K in a temperature range of 0–50 0C, that of lightweight concrete decreased to 0.5– 0.6

W/m K (63–70%) when AE agent was not used, and decreased further to 0.3–0.4 W/m

K (52–55%) with the addition of AE agent up to 1.5%. The thermal conductivity has a

strong relationship with the denseness of the matrix in materials and is defined as

Where λsm is the thermal conductivity of the solid matrix, λair is the thermal

conductivity of air, p is the porosity in the material, and τdry is the tortuosity factor of a

material in an O.D. condition. A detailed description of this conceptual model has

been presented in a previous paper. In accordance with the experimental results it was

shown that the value of the tortuosity factor slightly increased or remained constant

with an increase in porosity, when the pores were uniformly distributed in a solid

matrix. Considering the much lower thermal conductivity of air (0.02–0.03 W/m K)

relative to that of ceramic materials (1–50 W/m K), Eq. denotes that the thermal

38

conductivity of materials is high when the porosity in the matrix is high. In addition,

as shown in Fig. 2, the thermal conductivity linearly decreased as the porosity

increased, regardless of the location of the pores, which affects the compressive

strength and dynamic modulus of elasticity. It was inferred that the thermal

conductivity was not sensitively affected by the materials consisting of texture around

the pores.

Fig 1. Thermal conductivity of normal and lightweight concrete.

Fig 2. Relationship between porosity and thermal conductivity.

Conclusion:

39

The thermal conductivity of concrete was decreased when normal aggregates were

replaced by lightweight aggregates, and further decreased with addition of AE agent.

Moreover, the thermal conductivity linearly decreased as the porosity increased,

regardless of the location of the pores, which affects the strength and modulus of

elasticity.

Case 9:

INFLUENCE OF MOISTURE ON HEAT CONDUCTIVITY COEFFICIENT OF

AERATED CONCRETE [19][16][8]

The aim of the research was to measure heat conductivity coefficient of the aerated

concrete of the following apparent densities - 400 and 600 kg/m3 in dry state but also

in various moisture states.

For the examinations the following equipment and materials were used:

• Climatic chambers system for simulation of environmental conditions at the opposite

sides of the sample (Fig. 1) [4],

• TDR equipment for both reflectometric moisture determination in building materials

and the temperature - with analog temperature sensors placed in the TDR LP/mts

probes and digital temperature sensors - Dallas type,

• Peltier plates - used as heat flux probes,

• Multimeter for voltage readouts from the Peltier plates,

• Aerated concrete samples of the following apparent densities - 400 and 600 kg/m3

Aerated concrete 400 is characterized by smaller

value of the heat conductivity coefficient than aerated concrete 600. It could be

40

explained by smaller apparent density with the similar compound of solid phase. But

the λmst coefficient (unit rise per % vol.) is greater. Heat conductivity coefficient in dry

state for aerated concrete 400 is about 6 times smaller than in saturation and for

concrete 600 only 3.9 times.

It should be also underlined that the aerated concrete

is strongly prone to water influence, but comparing with red brick for example, time

of water desorption is longer. This fact, together with the increase of heat conductivity

coefficient due to moisture rise leads to the increased heat loses during winter season

and the decrease of heat-moisture comfort and indoor air microclimate.

Conclusion

41

Autoclaved aerated concrete is further considered a sustainable building product

because of its excellent insulating qualities resulting in increased energy efficiency.

AAC’s thermal efficiency stems from three factors. First, AAC structures result in

solid wall construction with integrated insulation. Entire wall coverage prevents the

thermal bridging associated with conventional stud-framed walls, which essentially

leave cold gaps around every stud and header.

Second, the solid wall construction of AAC

structures creates an airtight building envelope, minimizing uncontrolled air changes

while helping maintain desired indoor temperatures and maximizing the efficiency of

HVAC equipment.

Third, AAC structures benefit from the added

value of thermal mass and low thermal conductivity. The benefits of thermal mass,

however, vary by location, and are greatest in regions where the outdoor temperature

fluctuates above and below the desired indoor temperature over a 24-hour period.

AAC has excellent thermal insulating properties due to its cellular microstructure. The

thermal conductivity of foam concrete of density 1000 kg/m3 is reported to be one-

sixth the value of typical cement–sand mortar.

Comparison with normal concrete: The thermal

conductivity values are 5–30% of those measured on normal weight concrete and

range from between 0.1 and 0.7 W/mK for dry densities values of 600–1600 kg/m3,

reducing with decreasing densities . Thermal insulation of brick wall can be increased

by 23% when inner leaf is replaced with AAC of unit weight 800 kg/m3.

Effect of density variation on thermal conductivity:

Insulation is more or less inversely proportional to density of concrete. A decrease of

concrete dry density by 100 kg/m3 results in a reduction of thermal conductivity by

0.04 W/mK of lightweight aggregate concrete. Altering the mortar/foam ratio affects

density which has enormous impact on insulation capacity.

Influence of fly ash, light weight aggregate: 12–38%

reduction in thermal conductivity of foam concrete with 30% PFA (Pulverized Fuel

42

Ash) as compared to mixes with only Portland cement as binder is attributed to the

lower density and cenospheric particle morphology of fly ash particles, which

increases the heat flow path. AAC concrete exhibited typical thermal conductivity

between 0.23 and 0.42 W/mK at 1000 and 1200 kg/m3 dry densities. The replacement

of cement by finer fly ash (30% by weight of cement) helped to reduce temperature

development during heat of hydration. The use of lightweight aggregates with low

particle density in combination with artificially introduced air-voids in the mortar

matrix has been observed to be advantageous in reducing thermal conductivity. By

moderate filling of porous mortar with polystyrene granules, foam concrete of density

range 200–650 kg/m3 with thermal conductivity

0.06–0.16 W/mK could be produced.

Effect of temperature on thermal conductivity: Thermal

insulation is reported to improve with a reduction in temperature. While studying the

potential of cellular concrete for load bearing insulations for cryogenic applications,

Richard et al. reviewed the thermal and mechanical characteristics of foam concrete.

Influence of temperature variations from 22 to -196 0C is reported for selected

densities between 640 and 1440 kg/m3. An apparent reduction of 26% in thermal

conductivity of foam concrete has been reported when temperature was lowered from

22 to -196 0C.

The thermal conductivity investigations at micro level indicates that

(i) The method of pore-formation and curing plays a significant role in its

microstructure and properties.

(ii) (ii) The properties are influenced by the density and thus, they should be

qualified with the density, which is to be specified along with the moisture

content;

(iii) (iii) The chemical composition varies with the method of curing -

tobermorite gel being formed for moist-cured products and a more stable

form of tobermorite on autoclaving

43

The presence of tobermorite gel has significant effect on the thermal property of

AAC. The thermal insulating property is very superior.

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